HookThe man with a window into his own stomach
On 6 June 1822, a musket discharged by accident in a fur-trading post on Mackinac Island tore a hole in the stomach of a 19-year-old voyageur, Alexis St Martin. He survived — but the wound healed into a permanent opening, a fistula, straight into his stomach. The US Army surgeon William Beaumont spent the next decade doing something no one had managed before: tying pieces of food to a silk thread, lowering them through the hole, and pulling them out at intervals to watch digestion happen in real time. He proved that the stomach's juice was acidic and chemically dissolved food, not merely churned it.
Beaumont had stumbled onto the theme of the whole of B2: organisation. A meal is broken down by enzymes into molecules small enough to cross the gut wall; those molecules are carried by a transport system — heart, vessels and blood — to every cell in the body; and when that system, or the tissues it feeds, go wrong, you get the non-communicable diseases that fill UK hospital wards. This section reads the body as a hierarchy — cells build tissues, tissues build organs, organs build systems — in animals and in plants alike.
ModelThe organisation hierarchy — and the enzymes that run it
Living things are organised in levels: cells build tissues, tissues build organs, organs build organ systems, and organ systems build the organism. A tissue is a group of similar cells working together — muscular, glandular or epithelial tissue, for example. An organ is several tissues performing a function: the stomach contains muscular tissue to churn, glandular tissue to make juices, and epithelial tissue lining it. An organ system is organs working together, such as the digestive system.
Running almost everything are enzymes — biological catalysts that speed up reactions without being used up. Each enzyme has an active site shaped to fit one particular substrate, the 'lock-and-key' model, which is why enzymes are so specific. Digestion is enzymes chopping large, insoluble food molecules into small, soluble ones the body can absorb.
Enzymes are fussy about conditions. Reaction rate rises with temperature up to an optimum, then falls sharply as the enzyme denatures: the active site changes shape and the substrate no longer fits. Each enzyme also has an optimum pH — stomach protease works in acid, small-intestine enzymes in slightly alkaline conditions. Note the wording: a denatured enzyme is not 'killed', because enzymes were never alive.
MechanismDigestive enzymes and bile
Three enzyme families do the chemical work. Carbohydrases, such as amylase, break starch down into sugars, and are made in the salivary glands, pancreas and small intestine. Proteases break proteins into amino acids — the stomach's protease works in acid — and come from the stomach, pancreas and small intestine. Lipases break lipids into fatty acids and glycerol, made in the pancreas and small intestine.
The small, soluble products are then absorbed into the blood through the wall of the small intestine, which is beautifully adapted for it: villi give an enormous surface area, the wall is thin, and a rich blood supply keeps the concentration gradient steep.
Bile is the piece students most often misplace. It is made in the liver, stored in the gall bladder and released into the small intestine, and it does two non-enzyme jobs. It is alkaline, so it neutralises the acid arriving from the stomach and gives intestinal enzymes their optimum pH. And it emulsifies fats — breaking large fat droplets into many tiny ones — which massively increases the surface area for lipase to work on. Bile speeds up digestion, but it contains no enzymes and breaks no chemical bonds itself.
DataRequired practicals 4 and 5 — food tests and enzyme rate
Required practical 4 is four colour tests, and the marks are in learning them exactly. Iodine solution tests for starch: orange-brown turns blue-black. Benedict's solution tests for reducing sugars: heat in a water bath and blue turns green, then yellow, then brick-red as sugar increases. Biuret solution tests for protein: blue turns purple/lilac. For lipids, the ethanol emulsion test gives a cloudy white layer. Each test needs a control and the correct colour change remembered precisely.
Required practical 5 investigates the effect of pH on amylase. Mix amylase, starch and a buffer set to one pH; every 30 seconds, take a drop and add it to iodine on a spotting tile. When the iodine stays orange-brown, all the starch has been digested — record that time. Repeat across a range of pH values, using buffers to hold each pH constant.
The key move is turning time into rate. A faster reaction takes less time, so rate is the reciprocal of time. Plot rate against pH and the optimum is the peak of the curve — where amylase digests starch fastest.
At pH 6 the starch disappears in 60 s; at pH 7 in 40 s; at pH 8 in 120 s. Rate is \[\text{rate} = \dfrac{1}{\text{time}}.\] pH 6: \(1/60 = 0.017\ \text{s}^{-1}\). pH 7: \(1/40 = 0.025\ \text{s}^{-1}\). pH 8: \(1/120 = 0.008\ \text{s}^{-1}\). The highest rate is at pH 7, so that is this amylase's optimum. Beware the trap: the shortest time is the fastest rate, so reading times straight off the table draws the graph upside down — always convert to \(1/\text{time}\) first, and quote the unit as reactions per second.
MechanismThe heart and blood vessels
Humans have a double circulatory system: the heart pumps blood to the lungs to collect oxygen and drop off carbon dioxide, back to the heart, then out to the rest of the body — two circuits, so the blood is repressurised before its long trip round the body. The heart has four chambers. The right atrium and right ventricle send deoxygenated blood to the lungs via the pulmonary artery; the left atrium and left ventricle send oxygenated blood from the lungs out to the body via the aorta. Valves prevent backflow, and the coronary arteries supply the heart muscle itself.
The natural resting heart rate — around 70 beats per minute — is set by a group of cells in the right atrium acting as a pacemaker. A faulty rhythm can be corrected with an artificial electrical pacemaker.
Three vessels do three jobs. Arteries carry blood away from the heart at high pressure, with thick, muscular, elastic walls and a narrow lumen. Veins return blood at low pressure, with thinner walls, a wide lumen and valves to stop it flowing backwards. Capillaries are just one cell thick, so substances diffuse in and out quickly. The reliable trap: arteries carry blood away from the heart — usually oxygenated, but the pulmonary artery carries deoxygenated blood to the lungs.
ModelBlood — four components, four jobs
Blood is a tissue made of four parts. Plasma is the straw-coloured liquid that transports nearly everything dissolved: carbon dioxide to the lungs, urea to the kidneys, digested food, hormones and heat around the body.
Red blood cells carry oxygen. They are packed with haemoglobin, which binds oxygen in the lungs to form oxyhaemoglobin and releases it in the tissues. Two adaptations help: they are biconcave discs (large surface area for gas exchange) and have no nucleus (more room for haemoglobin).
White blood cells defend the body — some engulf pathogens by phagocytosis, others produce antibodies. Platelets are small cell fragments that trigger clotting, sealing wounds so pathogens cannot get in and blood is not lost. Knowing which component does which job is worth easy marks, because exam questions love to hand you a symptom (low haemoglobin, poor clotting) and ask which part is affected.
CaseCoronary heart disease and how we treat it
In coronary heart disease (CHD), layers of fatty material called atheroma build up inside the coronary arteries, narrowing them and reducing blood flow, so the heart muscle gets less oxygen — causing chest pain and, if an artery blocks completely, a heart attack. CHD is a non-communicable disease and, for decades, the UK's leading cause of death.
Two treatments dominate. Stents are mesh tubes inserted to hold a narrowed artery open, restoring blood flow — fast-acting and effective, but the surgery carries risk and stents can trigger further clotting. Statins are drugs taken long-term that lower blood cholesterol, particularly LDL, slowing atheroma build-up; roughly 7 to 8 million adults in England take one. They act slowly, must be taken for life, and cause side effects in some people.
Other repairs exist. Faulty heart valves can be replaced with biological or mechanical valves. In heart failure, a donor heart transplant is the fix, and an artificial heart can keep a patient alive while they wait — avoiding rejection, but risking blood clots. Every evaluation answer in this area is a benefit-versus-risk balance, which is exactly what examiners reward: state a benefit, state a risk, then judge.
CaseHealth, lifestyle risk factors and cancer
Health is a state of physical and mental wellbeing, not merely the absence of disease, and diseases interact: a weakened immune system leaves you open to other infections, some viruses trigger cancers, immune over-reactions cause allergies, and severe physical illness can trigger mental illness. Diseases are either communicable (spread by a pathogen) or non-communicable (not spread — the long-term conditions such as CHD, Type 2 diabetes and cancer).
Lifestyle risk factors are linked to non-communicable disease. A poor diet and lack of exercise raise the risk of obesity, cardiovascular disease and Type 2 diabetes; smoking damages the lungs and heart and harms unborn babies; alcohol damages the liver and brain; and carcinogens and ionising radiation raise cancer risk. The exam's sharpest point is here: a risk factor shows a correlation, but proving causation needs a mechanism — which is why 'smoking is linked to cancer' had to become 'tar in cigarette smoke is a carcinogen'.
Cancer is uncontrolled cell division — mitosis gone wrong — forming a tumour. Benign tumours stay in one place and are usually not dangerous. Malignant tumours invade neighbouring tissues and spread through the blood to form secondary tumours elsewhere (metastasis), and that spread is what makes cancer lethal. Risk factors include smoking, obesity, UV exposure, some viruses and inherited genes.
ModelPlant tissues and the plant organ system
A leaf is a plant organ built from several tissues. The upper epidermis is transparent to let light through. Just beneath, the palisade mesophyll is packed with chloroplasts for photosynthesis. Below that, the spongy mesophyll has air spaces for gas exchange. Running through as veins are the transport tissues, xylem and phloem, and the lower epidermis carries the stomata.
Two transport tissues carry different cargoes in different directions. Xylem carries water and dissolved minerals upward from the roots; its cells are dead, hollow and strengthened with lignin. Phloem carries dissolved sugars made in the leaves to wherever they are used or stored, up or down the plant — a process called translocation — and its cells are living. Root hair cells give a huge surface area for absorbing water and minerals from the soil.
Gas exchange and water loss are controlled by the stomata, tiny pores opened and closed by guard cells, trading a gas-exchange opening against losing water. Water evaporating from the leaves pulls the whole column of water up the xylem — the transpiration stream. The rate of transpiration rises with higher temperature, brighter light, more air movement and lower humidity.
A student uses a potometer and finds a leafy shoot takes up 0.8 cm³ of water in 20 minutes. The rate is \(0.8 \div 20 = 0.04\ \text{cm}^3\ \text{per minute}\). Moving a lamp closer raises the light intensity, the stomata open wider, and uptake climbs to 1.5 cm³ in the same 20 minutes — a rate of \(0.075\ \text{cm}^3\) per minute, almost double. A potometer strictly measures water uptake, but that is a fair proxy for transpiration because the vast majority of water a plant absorbs is lost through the leaves. Control temperature and air movement, though, or you cannot claim that light caused the change.
VocabularyKey terms the mark scheme pays for
TrapsMisconceptions that cost marks
ExamWhat examiners want
For enzymes, always name the enzyme, its substrate and its product together — 'amylase breaks starch into sugars' — and say where it is made. On a rate-against-temperature or pH graph, describe the rise to the optimum and then the fall due to denaturation, and use the word 'denatured', never 'killed'. Where a practical yields times, convert to rate as one over time before you draw or read the graph.
'Compare' questions on structure pairs — arteries versus veins, xylem versus phloem, benign versus malignant tumours — want matched points: thick wall against thin wall, high pressure against low, stays put against spreads. For coronary heart disease and lifestyle factors, the marks are in a balanced evaluation: give a benefit and a risk, then judge. On risk-factor data, separate correlation from causation explicitly — spotting that a link is not proof is the point being tested.
Required-practical answers reward exact colour changes (Benedict's blue to brick-red; Biuret blue to purple; iodine orange-brown to blue-black) and the reciprocal step for enzyme rate. For plant questions, the whole game is structure-and-function: name the tissue, state its adaptation, then attach its job — palisade mesophyll is near the top and full of chloroplasts, which is why it does most of the photosynthesis.